1. Field of the Invention
This invention relates to an optical fiber cable.
2. Prior Art
A conventional optical fiber cable 10e shown in FIG. 1 comprises an elongated reinforcing core 11, a plurality of optical fibers 18a spirally wound around the reinforcing core 11, and a sheath 13 covering the optical fibers, the cross-sectional shape of the cable being symmetrical with respect to a longitudinal axis of the cable.
It has been proposed to loosely mount a bundle of optical fibers 18a, tied together by yarns or the like into a generally circular cross-sectional shape, in a pipe 12e of a plastics material to provide an optical fiber cable 10f as shown in FIG. 2. Another alternative is to loosely mount a plurality of juxtaposed optical fibers 18a in the pipe 12e to provide an optical fiber cable 10g as shown in FIG. 3. Either of the bundle of optical fibers 18a and the juxtaposed optical fibers 18a are held in contact with the inner peripheral surface of the pipe 12e. These methods are advantageous in that the manufacture of the optical fiber cables 10f and 10g can be simplified since the optical fibers 18a are not wound around a reinforcing core as is the case with the first-mentioned conventional cable 10e.
When the optical fiber cable 10e of a symmetrical construction shown in FIG. 1 is to be wound around a drum, there is no need to take the upper and lower sides of the cable into consideration. However, when the optical fiber cable 10f, 10g of either FIG. 2 or FIG. 3, is to be wound around a drum, it is necessary to take the upper and lower sides of the cable into consideration. Particularly, a serious problem arises that the end of the optical fibers 18a can be displaced with respect to the end of the pipe 12e either when the optical fiber cable is transferred between two drums of different diameters or when the optical fiber cable is unwound from the drum so as to be extended.
For example, if the optical fibers 18a and pipe 12e of the optical fiber cable 10f of FIG. 2 have substantially the same length when the optical fiber cable is extended straight, and when the optical fiber cable is wound around a drum, the fiber bundle is displaced relative to the pipe 12e an amount of 2.pi.(l.sub.2 -l.sub.1) per one turn of the optical fiber cable. And, then, if the extended end of the fiber bundle is cut off to be brought into agreement with the end of the pipe 12e and is fixedly secured thereto and if the optical fiber cable is unwound from the drum, the end of the fiber bundle tends to be displaced inwardly longitudinally with respect to the end of the pipe, so that the fiber bundle is elongated and is subjected to undue pressure or tension, which results in an increased transmission loss.
When the optical fiber cable 10f of FIG. 2 is bent as shown in FIG. 4, the inner side of the pipe 12e is contracted while the outer side is extended or elongated, and there exists a neutral axis between the inner side and outer side of the pipe 12e at which neutral axis there occures no contraction and extension. Since the peripheral wall of the pipe 12e of a circular cross-section has a uniform thickness, the neutral axis coincides with a longitudinal axis 0 of the pipe 12e. However, in the case where the pipe 12e is modified in such a manner that the cross-sectional shape of the pipe 12e is not symmetrical with respect with the longitudinal axis, the neutral axis of the pipe is displaced from the axis. Therefore, if the optical fibers 18a are disposed so as to include the neutral axis, the optical fibers are not displaced longitudinally with respect to the pipe even when the optical fiber cable is wound around and unwound from the drum.